Prostatic Acid Phosphatase is a prostate-specific antigen expressed by both normal and malignant prostate tissue. To explore the potential role of xenoantigen immunization in prostate cancer, we initiated a phase I/IIa trial using dendritic cells (DC) pulsed with recombinant mouse PAP as a tumor vaccine. Eligible patients had recurrent prostate cancer, rising serum PSA, and no recent or concurrent changes in therapy.